(1) Field of the Invention
The present invention relates to an image forming apparatus such as a copier, printer or the like including a thermal fixing device in which the surface of the fixing roller is cleaned by a cleaning web.
(2) Description of the Related Art
Conventionally, there has been a known thermal fixing device used in image forming apparatuses such as copiers etc., which include a cleaning web type-cleaning device as a mechanism for reducing residual toner left over on the fixing roller surface.
As this cleaning device, a cleaning device that continues rotating a cleaning web (cleaning means) for a fixed time after the end of printing and then stops it in order to remove residual toner on the fixing roller surface, has been proposed (Japanese Patent Application Laid-open H2-284184).
However, the fixing device with the above cleaning device entails the problem that when print media (printing sheets) are continuously passed through the fixing device, a large amount of offset toner stagnates around the contact area where the cleaning web and the fixing roller contact each other (see FIG. 2 described below) because of lowering of the surface temperature of the fixing roller. As a result, the thus stagnant toner is fully heated and fused, and the fused toner weaves through the contact area to thereby dirty the paper.
To deal with this, in order to prevent stagnant toner 153 from weaving through the contact area even when a large amount of offset toner has stagnated around the above contact area, a cleaning device having a pressure changing means for varying the pressing force of the pressing member (pressing roller 136; see FIG. 1 below) that presses the cleaning web against the fixing roller has been proposed (patent document 2: Japanese Patent Application Laid-open 2003-167465).
Now, the basic configuration and operation of a thermal fixing device equipped with a conventional cleaning device will be briefly described.
FIG. 1 is an illustrative view schematically showing the configuration of a cleaning type-thermal fixing device.
As shown in FIG. 1, a thermal fixing device 121 includes a pair of a fixing roller 123 and a pressure roller 124.
Fixing roller 123 is composed of a core member formed of metal such as aluminum or the like having an elastic layer on the outer peripheral surface thereof, a pair of halogen lamp heaters, namely main and sub heaters 128 and 129 arranged inside the core member. A temperature detector 130 of a non-contact type thermistor is arranged around fixing roller 123 so that the surface temperature can be controlled within the range of 160 to 200 deg. .C by main heater 128 and sub heater 129.
Arranged above fixing roller 123 is a web-type cleaning device (cleaning unit) 120 for wiping off the residual toner adhering on the outer peripheral surface. Cleaning unit 120 is essentially comprised of a web sheet delivering roller 132, a tension roller 134 for applying a predetermined tension on a web sheet 133 delivered from web sheet delivering roller 132, a pressing roller 136 for pressing web sheet 133 against the outer peripheral surface of fixing roller 123 and a take-up roller 137 for taking up the used web sheet.
An unillustrated pressure changing mechanism (pressure changing means) is provided for the roller shaft of the pressing roller 136 so as to move and adjust the roller shaft of pressing roller 136 in the directions of a bidirectional arrow A, whereby the web nip width, designated at 139, formed between the fixing roller 123 surface and pressing roller 136 is varied to thereby change the pressing force against fixing roller 123. For example, when pressing roller 136 is moved in the upward direction of bidirectional arrow A to the position indicated by the dashed line, the pressing force lowers so that web nip width 139 becomes smaller. In contrast, when the roller is moved in the downward direction of bidirectional arrow A to the position indicated by the solid line, the pressing force increases so that web nip width 139 becomes greater.
Here, as the pressure changing means, it is possible to control the pressing force by using, for example a rotatable cam mechanism or the like. In addition, this cleaning unit 120 is a periodical replacement part to be replaced in a predetermined replacement cycle.
Similarly to fixing roller 123, pressure roller 124 also is composed of a metal core member having an elastic layer on the outer peripheral surface thereof, a halogen lamp heater, namely pressure roller heater 138 arranged in the core member.
Separation claws 135A and 135B are arranged to abut on the outer peripheral surfaces of fixing roller 123 and pressure roller 124, respectively so as to prevent the print paper, after the heating process, from winding around fixing roller 123 or pressure roller 124.
According to thermal fixing device 121 that functions as described above, as shown in FIG. 2, even when printing sheets are continuously passed through and hence the amount of offset toner becomes greater due to lowering of the surface temperature of fixing roller 123, it is possible to prevent stagnant toner 153 around the contact area from passing therethrough by enhancing the contact pressure of pressing roller 136 against fixing roller 123. As a result, it is possible to prevent leakage toner 150 and 151 from dirtying fixing roller 123 and printing paper by adherence of toner thereto.
Here, when printing sheets printed with high density patterns or high coverage patterns are passed through the fixing device, toner is liable to stagnate on the upstream side of the contact area between fixing roller 123 and cleaning web 133. In this situation, once printing is ceased, the surface temperature of fixing roller 123 lowers and the large amount of residual toner solidifies on the fixing roller 123 surface and adheres thereto.
To deal with this, in the conventional method of controlling the contact pressure of pressing roller 136 against fixing roller 123 based on the information as to lowering of the surface temperature of fixing roller 123 only, it is possible to reduce the amount of toner remaining around the contact area by further enhancing the contact pressure of pressing roller 136 against fixing roller 123 from the normal contact pressure of a predetermined level to a higher level, or increasing the rotational rate of take-up roller 137 to increase the winding length of web sheet 133.
However, in both of these cases, the friction between pressing roller 136 and fixing roller 123 is increased, so that there occurs the problem that the fixing roller 123 surface is worn out excessively, needing frequent maintenance work of the entire cleaning device and hence needing high maintenance cost and others.